1. Field of the Invention
This invention generally relates to a rangefinding (or distance-measuring) device for a camera and, more particularly, to a rangefinding device for a camera, in which light projecting means for projecting near-infrared light to an object and light receiving means for receiving object-reflected (or catoptric object) light reflected from an object are provided in such a manner as to be nearly in parallel with each other.
2. Description of the Related Art
Hitherto, there have been proposed and put to practical (or actual) use various rangefinding devices of what is called the active type for camera, which measures a distance therefrom to an object by projecting light, such as near-infrared light, emitted by a light projecting (or emitting) device onto the object through a light projecting lens for autofocusing (AF) and by then leading light reflected from the object to a light receiving device through a light receiving lens for autofocusing (AF) and a prism.
For example, a rangefinding device for a camera, which is disclosed in the Japanese Unexamined Patent Publication No. 4-305608/1992 Official Gazette, is of the active type that has a reflecting means, such as a mirror and a prism, having a reflection surface for reflecting an optical path axis in a direction parallel to a base line drawn between a light projecting portion and a light receiving portion is provided in an optical system composing the light projecting and receiving portions, thereby leading the object-reflected light to the light receiving device.
Thereby, without decreasing a base line length of a rangefinding device, the placement of members, such as the rangefinding device, can be simply arranged in such a manner that other members and mechanisms provided in a camera do not interfere with the object-reflected light. Consequently, the effective utilization of a space in the camera can be achieved. Further, because the base line length is not reduced, the accuracy in measuring a distance by a rangefinding device is not degraded.
However, according to technical means disclosed in the Japanese Unexamined Patent Publication No. 4-305608/1992 Official Gazette, in the case that the aforementioned reflecting means is formed with a prism, total reflection conditions determined by the reflection surface of this prism are not completely satisfied.
Namely, in the case that the aforesaid reflection means is formed with a prism, not all of the object light impinging upon the prism is reflected by the reflection surface of this prism onto the light receiving device, with the result that not all of the object light impinging upon the prism is led to the light receiving device. Thus, this conventional rangefinding device has a problem in that the light receiving accuracy of the light receiving element thereof is not good.
Further, in the case that the aforementioned reflecting means is formed with a mirror, the reflectance (factor) thereof is low and thus the device is not efficient. Consequently, it is difficult to ensure high rangefinding accuracy. This conventional device has problems in that serious reflection loss is incurred when the object is located far away therefrom and that it is difficult to secure the high rangefinding accuracy.
Thus, it has been suggested that, for instance, evaporation (or sputtering) of gold is performed on the reflection surface of the aforementioned reflecting means (such as a prism and a mirror) so as to increase the reflectance of the reflection surface thereof and enhance the rangefinding accuracy. However, in such a case, the manufacturing cost of the rangefinding device (or of the camera) increases.
Furthermore, in the case of employing the technical means described in the aforementioned Japanese Unexamined Patent Publication No. 4-305608/1992 Official Gazette, the optical path axis is reflected in a direction that is parallel to the base line connecting the light projecting portion with the light receiving portion, so that the flexibility in placing the light receiving element in the camera is lost. Therefore, such a layout is not advantageous in placing members in a camera. Namely, in the case of the conventional device, the efficient placement of members thereof in the camera cannot be achieved. In some cases, the size of the camera itself becomes large due to the placement of the members of the rangefinder device.